A variety of methods have been suggested for the manufacture of high-silica content glass articles, such as the single and double dispersion processes described by D. W. Johnson, et al. in Fabrication Of Sintered High-Silica Glasses, U.S. Pat. No. 4,419,115, and the process described by D. W. Johnson, et al in Sintered High-Silica Glass And Articles Comprising Same, U.S. Pat. No. 4,605,428. Uses of high-silica content include the fabrication of glass rods for use as preforms in the manufacture of optical fibers as suggested by F. Kirkbir, et alii, U.S. Pat. No. 5,254,508 for a Sol-gel Process For Forming A Germania-doped Silica Glass Rod, and the fabrication of secondary cladding tubes for use during fabrication of an optical fiber by a sol-gel process. Although sol-gel processes enable fabrication of glass objects at lower cost than other processes, N. Matsuo, et alii, in U.S. Pat. No. 4,680,046 for a Method Of Preparing Preforms For Optical Fibers, among others, has noted that it is difficult to provide a glass article that is large enough to be used as a preform for optical fibers.
Considering that the functioning part of an optical fiber (the core and inner cladding carrying 99+% of the optical energy) typically consists of but 5% of the mass, a significant part of this effort has concerned structures providing for overcladding of such inner portion. State of the art manufacture often makes use of an inner portion constituting core and inner clad region as fabricated by Modified Chemical Vapor Deposition, or, alternatively, by soot deposition in Outside Vapor Deposition or Vapor Axial Deposition. This core rod may be overclad by material of less demanding properties, and, consequently, may be produced by less costly processing. Overcladding may entail direct deposition on the core rod, or may result from collapsing an encircling tube. Such “overcladding” tubes have been produced from soot or fused quartz. Making very large bodies of soot require extensive processing, and large bodies of fused quartz are expensive.
It has been recognized that significant economies may be realized by fabricating overcladding tubes by sol-gel techniques. This well-known procedure is described, for example, in J. Zarzycki, “The Gel-Glass Process”, pp. 203–31 in Glass: Current Issues, A. F. Wright and J. Dupois, eds., Martinus Nijoff, Boston, Mass. (1985). Sol-gel techniques are regarded as potentially less costly than other known preform fabrication procedures. While sol-gel fabrication of overcladding tubes, and other optical glass components, has met with considerable success, improvements are continually sought.
From the earliest proposals for making large monolithic bodies of glass using sol gel techniques it was recognized that the sol gel process by its nature results in significant shrinkage from the gel state to the solid glass state. When the shape of the body and the overall dimensions are important, this presents a drawback to sol gel methods. Efforts to control the amount of shrinkage usually focus on reducing the water content of the sol. This is often expressed as the loading factor, i.e. the ratio of solid (glass powder) to liquid (water). Recently, we have had success in increasing the loading factor by using relatively large glass particles, and particles having spherical shape. Loading factors of over 75% can be obtained. See U.S. patent application Ser. No. 09/838,727, filed Apr. 19, 2001, which is incorporated herein by reference.
Another persistent problem in making very large sol-gel bodies, e.g. greater than 5 Kg, for state of the art optical fiber drawing is cracking of the gelled body. Cracking may occur during drying or handling of the gelled body prior to consolidation. See for example, T. Mori, et al, “Silica Glass Tubes By New Sol-Gel Method”, J. Non-Crystalline Solids, 100, pp. 523–525 (1988), who describe the cracking problem, and recommend modification of the starting mixture and of the gel forming process, both of which are involved and expensive. The cracking problem is explained in a paper by Katagiri and Maekawa, J. Non-Crystalline Solids, 134, pp.183–90, (1991) which states, “One of the most important problems in the sol-gel preparation method for monolithic gels is avoidance of crack formation which occurs during drying”. A 1992 paper published in the Journal of Material Science, vol. 27, pp. 520–526 (1992) is even more explicit: “Although the sol-gel method is very attractive, many problems still exist, as pointed out in Zarzycki. Of these problems, the most serious one is thought to be the occurrence of cracks during drying of monolithic gel”. The reference then reviews remedies, e.g. hypercritical drying procedures and use of chemical additives such as N,N dimethylformamide, collectively referred to as Drying Control Chemical Additives. Both methods are regarded as expensive and, therefore, undesirable in routine glass production. An extensive description of a suitable sol-gel process, and of additives useful for improving the strength of sol-gel bodies, is contained in U.S. Pat. No. 5,240,488, which is incorporated herein in its entirety.
The cracking problem becomes more severe as the size of silica articles, preforms in the case of commercial fiber production, increases. State of the art optical fiber manufacture typically involves drawing hundreds of kilometers of fiber from a single preform. These preforms typically exceed 10 Kg in size. Although improvements in techniques for making large sol-gel bodies have been made, strength continues to be an issue and any process modification that results in improvement in the strength of intermediate products during the sol-gel process will constitute a valuable contribution to the technology.